Backward-wave oscillator mixer



y 9, 1956 J. R. WHENNERY BACKWA D-WAVE OSCILLATOR MIXER 2 Sheets-Sheet Filed June 15, 1955 //v VEN TOR ATTORNEY JOHN R. WH/NNER I" By W WW I 0 l H .I um v Ml Ni 5 v IW LbFSo May 29, 1956 .1. R. WHINNERY BACKWARD-WAVE OSCILLATOR MI XER 2 Sheets-Sheet 2 Filed June 15, 1955 90 E E F TY R AC F N F ME O MW EA RE 6 mm M N R FREQUENCY OF OSCILLA TION.

FREQUENCY JOHN R. WH/NNEP);

//v VEN TOR TORNE Y United States PatentO' BACKWARD-WAVE OSCILLATOR MIXER John Roy Whinnery, Orinda, Califi, assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Application June 15, 1955, Serial No. 515,624

Claims. (Cl. 250-20) This'invention relates to microwave oscillator-mixer devices and more particularly to a wave-type oscillatormixer incorporating a single electron stream and capable of simultaneously developing a backward-oscillatory wave, amplifying an electromagnetic signal wave of a different frequency and mixing the two waves to produce an intermediate frequency signal.

.As is generally known, a space-harmonic slow-wave structure is capable of propagating an electromagnetic Wave having group and phase velocities that are in opposite directions. A wave of this type is generally designated as a backward wave. In the event that an electron stream is .employed'to amplify a backward-wave, it is directed through the region occupied by the wave in the direction of and at a velocity slightly greater than the phase velocity of the wave. In that the backward-wave has an inherently regenerative effect on the electron stream, backward-wave oscillations are produced when an electron stream of sufficient current is directed through the above type space-harmonic structure. On the other hand, it is possible to elfect backward-wave amplification by employing an electron stream of a current less than the start-oscillation current. For a given space-harmonic slow-wave structure having an electron stream directed therethrough at a predetermined velocity, backward-wave amplification occurs over a very narrow portion of the frequency spectrum which does not include the'frequency at which the backward-wave oscillations would occur. In addition, it has been found that backward-wave oscillations do not saturate the electron stream to the extent that backward-wave amplification is precluded.

In accordance with the present invention, a single electron stream device is adapted to operate as a microwave-oscilator-mixer by simultaneously effecting backward-wave oscillations and backward-wave amplification. This is accomplished by directing the electron stream through a space-harmonic slow-wave structure having a uniform periodicity and simultaneously launching a signal to be amplified therealong from the end farthest from that first entered by the electron stream. The periodicity of the slo-wave structure together with the velocity of the electron stream determines the location of the band of frequencies wherein backward-wave amplification will occur and the frequency of the backward-wave oscillations. It has been found that the dilference between the frequency of oscillation and the mean frequency of the band wherein amplification occurs remains efiectively constant in spite of substantial changes in the velocity of the electronstream, thus enabling the device to be electronically tuned .so as to amplify a desired signal frequency.

The aforementioned difference in frequency is what is generally known as the intermediate frequency. In order to produce an intermediate frequency signal, it is necessary to mix "the oscillator signal with the received signal. In accordance with one embodiment of the present invention, an intermediate frequency signal is produced by impressing the electromagnetic waves prop- 2,748,268 Patented. May 29, 1955 agated by the slow-wave structure and appearing at the end thereof first entered by the electron stream across a square-law detector such as, for example, a crystal rectifier. In an alternate embodiment of the invention, the intermediate frequency signal is derived from the modu lations of the electron stream after it has passed through the slow-wave structure. The apparent advantage of these devices over those presently in use is one of simplicity in that the local oscillator, mixer and amplifier are included in one tube. In addition, the frequency of the local oscillator remains in a fixed relationship with respect to the range wherein amplification occurs while being tuned. This tuning may be eifected by controlling the velocity of the electron stream.

It is therefore an object of this invention to provide a simple apparatus capable of simultaneously providing local oscillations, mixing with .a wave of different frequency and amplification.

Another object of the invention is to provide a wavetype oscillator-mixer wherein backward-wave oscillations and amplification are effected by means of a single electron stream.

Still another object of this invention is to provide an electronically tuned backward wave oscillator mixer wherein the frequency of oscillation automatically tracks with the received signal.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings in which several embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only, and are not intended as a definition of the limits of the invention.

Fig. l is a schematic representation of one embodiment of the invention together with associated circuitry;

Fig. 2 is a schematic representation of a portion of the device of Fig. 1 showing an alternate manner of producing an intermediate frequency signal; and

Fig. 3 is an explanatory graph showing the relationship between the frequency of oscillation, the range of frequencies wherein amplification occurs, and the intermediate frequency.

Referring now to the drawings, Fig. 1 illustrates an embodiment of the oscillator-mixer of the invention comprising a space-harmonic slow-wave structure 1!) with an input connection 12 for receiving ,a signal to be amplified at one extremity and having a square-law detector 14 at the other, an electron gun 16 disposed at said other end of the slow-wave structure for producing an electron stream, a solenoid 18 disposed about the slowwave structure It} for directing the electron stream therethrough, and a collector electrode 20 disposed at said one end of said slow-wave structure for intercepting and collecting the electron stream.

More particularly, the space-harmonic slow-wave structure 10 may, for example, constitute a folded rectangular waveguide, as shown in Fig. 1. Thus, structure 10 may comprise an oblong rectilinear chamber 22 having a series of periodically spaced conductive sheets24 disposed transversely across the chamber and protruding inwardly from alternate top and bottom sides of the chamber 22, as viewed in the drawing, to adistance equal to the spacing between adjacent conductive sheets 24 from the opposite side. Thus, the conductive sheets 24 convert the chamber 22 into a section of folded waveguide having a width equal to that of the chamber 22 :and a height generally equal to the spacing between the conductive sheets 24. Outer end portions 25, 26 of the chamber 22 together with the transverse conductive sheets 24 are each provided with an aperture disposed in the central portion of the chamber 22 to form a rectilinear path 28 lengthwise through the slow-wave structure. 10. Suitable openings are provided in the chamber 22 at the extremities of the slow-wave structure to enable the waveguide thus formed to be connected to the input connection 12 and the square-law detector 14. The waveguide forming the input connection 12 is provided with a dielectric seal 30 to subsequently enable the chamber 22 to be evacuated While still allowing the introduction of an electromagnetic signal wave.

The square-law detector 14, disposed at the end of slow-wave structure 10 nearest the electron gun 16, comprises a crystal rectifier 32 which is centrally disposed within the waveguide at a distance of approximately onequarter guide wavelength from a termination 33 which provides a conductive surface across the end of the waveguide. One terminal of the crystal rectifier 32 is connected to a center conductor 34 of a shorted one-half Wavelength coaxial stub 36 which produces a virtual shorting plane across the internal surface of the waveguide. Stub 36 is disposed external to the waveguide with its center conductor 34 in alignment with the crystal rectifier 32. A load resistor 38 is inserted in series with the center conductor 34 of the stub 36 intermediate its shorting termination and the crystal rectifier 32 to provide an impedance across which to develop the intermediate frequency signal. Directly opposite the coaxial stub 36, a coaxial output section 40 having a center conductor 42 is connected between one of the output terminals 44 and the nearest terminal or electrode of the crystal rectifier 32. A dielectric material is disposed between conductors 40 and 42 to provide a seal for the chamber 22. The outer conductor of coaxial output section 40 is referenced to ground which, in turn, maintains the entire slow-wave structure 10 at ground potential.

The electron gun 16 for producing the electron stream is housed within a glass bulb 46 which is sealed to the end portion 25 of the chamber 22 so as to enable it to be evacuated. Electron gun 16 comprises a cathode 50 with a heater 51, a focusing electrode 52 and an accelerating electrode 54. The cathode 50 has an electron emitting surface disposed in register with the path 23 and is energized by means of a battery 56 which is connected across the heater 51 and referenced to the cathode 50. During operation, the cathode 50 is maintained at an adjustable potential of the order of -3000 volts negative with respect to ground. This is accomplished by means of a connection therefrom to a tap 58 of a potentiometer 60 which is, in turn, connected across the terminals of a battery 62, the positive terminal of which is referenced to ground. The focusing electrode 52 is disposed adjacent the electron emitting surface of cathode 50 and provides a surface of revolution at an angle of approximately 67.5 about the path 28. Focusing electrode 52 is maintained at a potential equal to that of cathode 50 by means of a connection thereto. trode 54 is disposed intermediate the focusing electrode 52 and the end portion 25 of the chamber 22. This electrode 54 is disc-shaped and has an aperture through its central portion to allow passage therethrough of the electron stream. During operation, accelerating electrode 54 is maintained at a potential that is sufliciently positive so as to allow current in excess of the start-oscillation current to flow. This potential is of the order of several hundred volts positive with respect to ground and is impressed on electrode 54 by means of a connection therefrom to a battery 64, the negative terminal of which is referenced to ground.

The solenoid 18 is disposed concentrically about and coextensive with the path 28. A current of sufficient magnitude to produce a longitudinal magnetic field of-the order of from 600 to 1000 gauss along the path 28 to focus and constrain the electron stream therealong is Accelerating eleccaused to flow through the solenoid 18 by means of connections across the terminals of a battery 66.

The collector electrode 20 is disposed external to the chamber 22 at the end of the path 28 farthest from the electron gun 16 so as to intercept and collect the electron stream. In order to avoid undesirable secondary electron emission effects, the electrode 20 is maintained several hundred volts positive with respect to the potential of slow-wave structure 10 which is at ground by virtue of the connection thereto from the outer conductor of coaxial section 40. This is accomplished by a connection from the electrode 20 to the positive terminal of a battery 68, the negative terminal of which is referenced to ground. In addition, the collector electrode 20 is housed in a glass bulb 70 which is sealed to the end portion 26 of chamber 22 thereby enabling the bulbs 46, 70 and the chamber 22 to be evacuated.

In the operation of the device of the present invention, a signal of frequency f1 is introduced through the input connection 12 to the end of the space-harmonic slowwave structure 10 farthest from the electron gun 16. As is generally known, a backward-wave amplifier will only amplify a comparatively narrow portion of the frequency spectrum such as is indicated by the curve in Fig. 3 which shows a plot of output signal amplitude versus frequency. The mean frequency of the amplified portion of the frequency spectrum is dependent upon the periodicity of the space-harmonic slow-wave structure together with the velocity of the electron stream. The velocity of the electron stream is, of course, determined by the potential predominating throughout the path 28 relative to that of the cathode 50. Normally this will be the difference in potential between the slow-wave structure 10 and the cathode 50. In that the periodicity of the slow-wave structure is fixed, the mean frequency of the amplified portion of the frequency spectrum may be tuned by controlling the velocity of the electron stream, which is accomplished by varying the potential impressed on the cathode 50 by means of tap 58. Thus, the position of the tap 58 of potentiometer 60 is adjusted so that the mean frequency of the amplified portion of the spectrum coincides with the frequency ii of the introduced signal, as shown in Fig. 3. Under these circumstances, the electromagnetic signal wave will increase in magnitude as it is propagated by the space-harmonic slowwave structure 10 in a direction opposite to the direction -of flow of the electron stream.

In addition to the above, an electron stream current that is greater than the current required to start oscillations is caused to flow by impressing a predetermined potential on the accelerating electrode 54. In this manner backward-wave oscillations are produced whereby a backward oscillatory wave having an associated frequency f0 and a magnitude comparable to that of the signal wave is generated within the slow-wave structure 10 and propagated thereby along with the signal wave. The relative magnitude and the frequency of this wave is represented by the line 82 in the plot of Fig. 3. It has been found that an electron stream employed to generate backwardwave oscillations at frequency f0 does not saturate in the usual manner and hence does not affect the amplification 'of the signal Wave of frequency f1 introduced at the input connection 12.

As previously stated, the difference between the frequencies f0 and f1 is the intermediate frequency. This intermediate frequency remains substantially constant for any frequency f1 throughout a band. Throughout the S band this intermediate frequency is of the order of 30 mo. and in the case of. the X band, is of the order of me. An intermediate frequency signal is developed across the resistor 38 by impressing both waves of frequency f1 and f0 across the square-law detector 14 and is available at the output terminals 44.

7 Alternatively, an intermediate signal may be derived directly from the electron stream as shown in Fig. 2.

Referring to this figure, the battery 68 which maintains the collector electrode 20 at several hundred volts positive with respect to ground is replaced by a parallel resonant circuit 90 comprising a capacitor 92 and an inductor 94 which is resonant at the intermediate frequency. In this manner, a load impedance is produced in the path from collector electrode 20 to ground only for frequencies in the vicinity of the intermediate frequency. Any signal appearing across the parallel resonant circuit 90 is coupled to stage 96 for amplification and then made available at the output terminals 100.

In the latter case, the intermediate signal is derived from the modulations of the electron stream as they exist when intercepted by the collector electrode 20. The electromagnetic waves of frequency f1 and f0 interact non-linearly with the electron stream while being propagated through space-harmonic slow-wave structure and, accordingly, produce concomitant modulations in the electron stream at the intermediate frequency. These modulations constitute, in effect, signal current at the intermediate frequency. Upon being intercepted by the collector electrode this signal current flows through the parallel resonant circuit 90 which presents a load impedance in the vicinity of the intermediate frequency. An intermediate frequency signal is thus derived from the electron stream which is amplified by an amplifier 96 and made available at the output terminals 100.

The intermediate frequency in this latter case may be slightly different than in the device of Fig. 1 because of the possible difference in, the group and phase velocities of the backward-oscillatory and the signal Waves. Any difference, however, would be nominal and the intermediate frequency would remain substantially constant throughout any one band of frequencies.

What is claimed is:

1. A wave-type microwave oscillator-mixer device comprising a space-harmonic slow-wave structure capable of propagating electromagnetic waves along a predetermined path at velocities substantially less than the velocity of light, means for launching an electromagnetic signal wave on said slow-wave structure along said path in one direction; means disposed at one end of said slow-wave structure for producing an electron stream; and means for directing said electron stream along said path through said slow-wave structure in a direction opposite to said one direction and at a velocity to effectively interact with said signal Wave and of a current to simultaneously produce backward-wave oscillations.

2. The wave-type microwave oscillatonmixer device as defined in claim 1 including additional means disposed at the end opposite from said one end of said slow-wave structure for deriving an intermediate frequency signal representative of said signal wave directly from said electron stream, said intermediate frequency being equal to the difference between the frequency of said signal wave and the frequency of said backward-wave oscillations.

3. The wave-type microwave oscillator-mixer device as defined in claim 2 wherein said additional means for deriving an intermediate frequency signal from said electron stream comprises an electrode for intercepting and collecting said electron stream, and a parallel resonant circuit connected between said electrode and a point of substantially fixed potential, said parallel resonant circuit being resonant at said intermediate frequency.

4. The wave-type microwave oscillator-mixer device as defined in claim 1 including a square-law detector disposed at said one end of said slow-wave structure, and means for simultaneously impressing said signal wave and said backward oscillatory wave across said detector to produce an intermediate frequency signal representative of said signal wave, said intermediate frequency being equal to the difference between the frequency of said signal wave and the frequency of said backward-wave oscillations.

5. A microwave oscillator-mixer comprising a waveguiding structure folded periodically about a predetermined path for propagating an electromagnetic wave therealong at a velocity substantially less than the velocity of light; means for launching an electromagnetic signal wave in said waveguiding structure along said path in one direction; means disposed at one end of said waveguiding structure for producing an electron stream; means for directing said electron stream along said path in a direction opposite to said one direction and at a velocit to effectively interact with said signal wave and of a current to simultaneously produce a backwardoseillatory wave; and detection means disposed at one end of said waveguiding structure responsive to said signal and backward-oscillatory waves for producing an output signal representative of said signal wave and having a frequency equal to the difference between the frequency corresponding to said signal wave and the frequency corresponding to said backward-oscillatory wave.

References Cited in the file of this patent UNITED STATES PATENTS 2,367,295 Llewellyn Jan. 16, 1945 2,641,730 Touraton et al June 9, 1953 2,654,047 Clavier Sept. 29, 1953 FOREIGN PATENTS 699,893 Great Britain Nov. 18, 1953 1,080,027 France May 26, 1954 OTHER REFERENCES Backward-Wave Tube, article by Heffner in Electronics, October 1953, pages to 137. 

